We propose to study event-related potential: (ERPS) and performance measures to delineate attentional dysfunction in Attention-deficit Hyperactivity Disorder (ADHD) adolescents. We will explore the extent of differences in the prevalence of deficits in processing varying degrees of cognitive load and demands of selective attention among ADHD patients identified in childhood a: having ADHD with hyperactivity as well as inattentiveness (ADHD/Combined Type), ADHD without hyperactivity (ADHD/Inattentive Type), ADHD/Combined Type with reading disorder (RD) ADHD/Inattentive Type with (RD), and RD without ADHD. Male: and files in each group will be sampled in sufficient numbers in order to examine hitherto neglected sex differences. In order to evaluate the differential impact of pharmacological therapy on these clinical populations we will study all these subjects in a double-blind challenge with methylphenidate (0.5 mg/Kg) and placebo. This research will provide information on the extent of impact of stimulants on attentional function among various forms of the ADHD syndrome. We will also test normal adolescents without medication, so that we can determine the extent to which the ADHD and RD samples attentional functions are normal.